1. Field of the Invention
The present general inventive concept relates to a mobile device, and more particularly, to an apparatus and method of supplying separate power voltage to a switching power amplifier stage in a variable manner in order to provide high output power and a high quality sound environment, and a mobile device using the same.
2. Description of the Related Art
Typically, mobile devices are internally provided with a D-class switching power amplifier and a loud speaker to play back audio signals. In the D-class amplifier, the audio signal changes into a pulse width modulation (PWM) signal in order to perform a switching operation. Also, the mobile devices adopt an external power supply unit such as a cradle and a secondary battery which is rechargeable.
A technology relating to a mobile device that uses a conventional external power supply unit illustrated in FIG. 1 is disclosed in Japanese Patent Document No. 2004-112377, entitled “Portable Information Terminal”, filed in Sep. 19, 1992.
Referring to FIG. 1, the portable information terminal uses a power voltage supplied from an internal battery B to drive a D-class amplifier 18 when a power voltage is supplied from an external power supply 36. The D-class amplifier 18 is driven by a low voltage from the battery B.
In contrast, the portable information terminal cuts off the battery voltage being supplied to the D-class amplifier 180 and supplies a high external voltage to the D-class amplifier 18 when the power voltage is not supplied from the external power supply 36. Accordingly, the D-class amplifier 18 is driven by a high voltage from an external power supply 360 (not illustrated).
Generally, the external power voltage for charging the battery B has a level relatively higher than the voltage of the battery B in consideration of safety. Therefore, the conventional portable information terminal may output an abrupt loud sound when the higher level voltage is supplied, and a user's hearing may be damaged accordingly. In addition, the power supply unit of the portable information terminal of FIG. 1 is susceptible to noises and cannot guarantee safety. Furthermore, although the conventional portable information terminal is provided with an internal loud speaker, the internal loud speaker cannot be driven by a high output power due to its structural characteristic.